Some embodiments described herein relate generally to methods and apparatus for improving the performance of a coherent optical transponder in an optical communication system. In particular, but not by way of limitation, some embodiments described herein relate to methods and apparatus for detecting and compensating bandwidth limitation and modulation nonlinearity of a coherent optical transponder in an optical communication system.
With a growing demand of optical communication systems with high data rates capability, optical quadrature amplitude modulation (QAM) signals are generated to provide high data-carrying capacity and high spectral efficiency. Quadrature amplitude modulation (QAM) is a modulation technique where two or more binary or multi-level electrical data signals are modulated, via an in-phase, or “I” channel, and a quadrature (90 degree) phase, or “Q” channel, onto a single optical carrier wave such that both the amplitude and the phase of the optical carrier wave are modulated with data to enhance the efficiency of the spectral occupancy. Other modulation techniques include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), differential quadrature phase-shift keying (DQPSK), and on-off keying (OOK). Polarization multiplexing (PM) is a multiplexing technique where two independent electrical data signals are first modulated onto an optical carrier wave having orthogonal polarizations (e.g., a first electrical data signal is modulated onto an X channel polarization and a second electrical data signal is modulated onto a Y channel polarization), then the signal on two polarizations are further multiplexed together through a polarization beam combiner so that the overall data throughput is doubled without doubling the spectral bandwidth.
A typical dual-polarization QAM (DP-QAM) transponder includes four tributary channels, XI, XQ, YI, and YQ, which are used for in-phase and quadrature modulation for both an X channel polarization and a Y channel polarization. Within each tributary, a bandwidth limitation may result from various components within the optical transponder, for example, the digital-to-analog converter (DAC), the radio frequency (RF) traces in the print circuit board (PCB), the pluggable interface (if applicable), the RF electrical amplifier, and/or the optical modulator. In addition to the bandwidth limitation, a modulation nonlinearity may result from a transfer function of the optical modulator and/or a nonlinear amplitude response from the various components within the optical transponder.
Known solutions to compensate the bandwidth limitation do not produce satisfying results for an optical transponder with a high baud rate (e.g., >400 G) or with a high amplitude signal. In addition, the known solutions often measure the coupled effects of the bandwidth limitation and the modulation nonlinearity and thus, the compensation lacks accuracy. Known solutions to compensate the modulation nonlinearity often compensate effects from either the transfer function of the optical modulator or the nonlinear amplitude response, and thus lack satisfying results.
Accordingly, a need exists for methods and apparatus to measure and compensate the de-coupled effects of the bandwidth limitation and the modulation nonlinearity of a high baud rate optical transponder. In addition, a need exists for methods and apparatus to compensate the modulation nonlinearity by compensating the transfer function of the optical modulator as well as the nonlinear amplitude response of the optical transponder.